I wrote a while ago about pain and how impossible it is to describe. http://diaryofabenefitscrounger.blogspot.com/2010/10/pain.html
I think the thing that really helped to convey it best was when hospitals started to allow patients to administer their own drugs. My bowel is usually so inefficient, that any medications I take orally might not have any effect, or they might work several hours after I've taken them, leaving me at risk of overdosing.
Over the years, I've learnt to give myself injections, either just under the skin (sub cut) or into a muscle (IM). When I have Crohn's pain, nothing matters but getting rid of it. I barely notice the bee-sting of the needle or the cramp as the liquid analgesia seeps deep into my muscle - the pain I'm already in means that I barely even notice.
When I started giving myself injections on the ward, without fail, the nurse who had bought it would tell me how brave I was. Despite 12 inch abscesses, perforated bowel, adhesions, obstructions and twisted guts, it was pushing a needle into my own flesh that they could empathise with. They could see it and imagine how it might feel.
Now, when they say I'm brave for giving myself injections, I just ask them to imagine how bad the pain in my gut must be if I'm willing to inject myself to get rid of it.